Fig. 35.12 1. GFR increases. GFR increases mainly as a result of the decrease in sympathetic nerve activity. Sympathetic fibers innervate the afferent and efferent arterioles of the glomerulus and control their diameter. Decreased sympathetic nerve activity leads to arteriolar dilation and an increase in renal plasma flow (RPF). Because the effect appears to be greater on the afferent arterioles, the hydrostatic pressure within the glomerular capillary is increased, thereby increasing the GFR. Because RPF increases to a greater degree than GFR, the filtration fraction (GFR/RPF) decreases. Natriuretic peptides also increase GFR by dilating the afferent arterioles and constricting the efferent arterioles. Thus the increased natriuretic peptide levels that occur during ECFV expansion contribute to this response. With the increase in GFR, the filtered load of Na+ increases.
2. The reabsorption of Na+ decreases in the proximal tubule and loop of Henle. Several mechanisms reduce Na+ reabsorption by the proximal tubule. Because activation of the sympathetic nerve fibers that innervate this nephron segment stimulates proximal tubule Na+ reabsorption, the decreased sympathetic nerve activity that results from ECFV expansion decreases Na+ reabsorption. In addition, angiotensin II directly stimulates Na+ reabsorption by the proximal tubule. Because angiotensin II levels are also reduced under this condition, proximal tubule Na+ reabsorption decreases as a result. Starling forces across the proximal tubule also change. The elevated hydrostatic pressure within the glomerular capillaries in ECFV expansion also tends to increase the hydrostatic pressure within the peritubular capillaries. In addition the decrease in filtration fraction reduces the peritubular oncotic pressure. These alterations in the capillary Starling forces (i.e., hydrostatic and oncotic) reduce the absorption of solute (e.g., NaCl) and water from the lateral intercellular space and thus reduce tubular reabsorption of NaCl (see for a complete description of this mechanism). Both the increase in the filtered load and the decrease in NaCl reabsorption by the proximal tubule result in delivery of more NaCl to the loop of Henle. Because activation of sympathetic nerves and angiotensin II and aldosterone stimulate NaCl reabsorption by the thick ascending limb of the loop of Henle, the reduced nerve activity and low angiotensin II and aldosterone levels that occur with ECFV expansion reduce NaCl reabsorption by the thick ascending limb. Thus the fraction of the filtered load delivered to the distal tubule is increased.